1. Field of the Invention
This invention relates to a particle detecting device used for counting the number of low-density particles in a flowing fluid and measuring the corresponding particle size distribution by irradiating the fluid containing the particles with a focused light beam and detecting scattered light of the light beam reflected by each particle.
2. Description of the Related Art
A particle detecting device that detects particles such as dust or cells is useful in processes for preparing semiconductors and pharmaceuticals, for example, in measuring the cleanliness of the atmosphere, the quality of purified water and medicines, etc. In medical sciences and biology, such a device is useful for testing the condition of cells. Recently, there has been a need for a particle detecting device which is capable of detecting fine particles.
FIG. 8 illustrates the structure of a conventional device for detecting particles in a fluid. Referring to FIG. 8, reference numeral 1 represents a flow-cell having a square cross-sectional shape and transparent side walls 1a, 1b and 1c. A sample fluid 2 containing the particles to be measured flows through flow-cell 1 in a direction perpendicular to the drawing sheet, i.e., out of the page.
Reference numeral 3 represents a light beam irradiating mechanism comprising an irradiator 5 which irradiates a plurality of parallel rays as a pencil beam 4 and a focusing lens 7 which focuses pencil beam 4 to form a light beam 6. Light beam irradiating mechanism 3 is adapted to irradiate sample fluid 2 in flow-cell 1 with light beam 6 through transparent side wall 1a of flow-cell 1.
For convenience of illustration and shown particularly in FIG. 9, a rectilinear coordinate system is used here having coordinate axes A.sub.1 -A.sub.2, B.sub.1 -B.sub.2, and C.sub.1 -C.sub.2 which intersect at an origin O. Origin O is located in the center of flow-cell 1. Axes B.sub.1 -B.sub.2 and C.sub.1 -C.sub.2 define a plane parallel to the sheet of paper on which FIG. 8 is drawn. Axis A.sub.1 -A.sub.2 passes through the origin perpendicular to the plane defined by axes B.sub.1 -B.sub.2 and C.sub.1 -C.sub.2. During irradiation of flow-cell 1 with light beam 6, the optical axis of light beam 6 is aligned with axis B.sub.1 -B.sub.2.
A portion of light beam 6 is scattered by the particles in sample fluid 2 in the vicinity of origin O when sample fluid 2 is irradiated by light beam 6. Reference numeral 8 represents a light receiving mechanism for receiving this scattered light. Light receiving mechanism 8 comprises a condensing lens 9 which condenses the scattered light, and a photoelectric converter 10 which converts the light condensed by lens 9 into an electrical signal 10a corresponding to the quantity of scattered light and outputs electrical signal 10a. Light receiving mechanism 8 further comprises an aperture member 11 having a circular aperture 11a for restricting the angle of the scattered light incident upon photoelectric converter 10. Aperture member 11 is disposed between condensing lens 9 and photoelectric converter 10. The optical axis of light receiving mechanism 8 is aligned with axis C.sub.1 -C.sub.2.
Reference numeral 12 represents a beam block capable of absorbing light beam 6 to prevent portions of light beam 6 which have penetrated into flow-cell 1 as stray light from entering into light receiving mechanism 8.
Reference numeral 14 represents a signal processing portion of light receiving mechanism 8 which processes electrical signal 10a to indicate the state of the particles in sample fluid 2, for example, the number of particles and the particle size distribution.
The operation of light receiving mechanism 8 of the particle detecting device shown in FIG. 8 will now be described with reference to FIG. 9, which provides an enlarged perspective view of principal components of the particle detecting device shown in FIG. 8. The rectilinear coordinate system described above is shown in FIG. 9 to comprise axes A.sub.1 -A.sub.2, B.sub.1 -B.sub.2, and C.sub.1 -C.sub.2, each intersecting at origin O and being mutually perpendicular. Axis A.sub.1 -A.sub.2 is aligned with the longitudinal axis of flow-cell 1. Reference character P represents an arrow which shows the direction of flow of sample fluid 2, the arrow P paralleling axis A.sub.1 -A.sub.2. Reference numeral 13 represents a cubic visual field of light receiving mechanism 8, which field is disposed in the vicinity of origin O. Visual field 13 is formed in a spindle shape, its axis being aligned with axis C.sub.1 -C.sub.2 and its center being at origin O. Light receiving mechanism 8 is as described above. Only the light irradiated by visual field 13 and captured by light receiving mechanism 8 is photoelectrically converted by light receiving mechanism 8 since visual field 13 is arranged within the visual field of light receiving mechanism 8. Referring to FIG. 9, the structure of the device as described above causes the entire portion of visual field 13 to be included in light beam 6. Scattered light in the form of light pulses is produced when the particles conveyed by sample fluid 2 pass through visual field 13. The scattered light pulses impinge upon photoelectric converter 10 and are photoelectrically converted into electrical signal 10a, which also takes the form of pulses. As a result, the number and size of the particles which pass through visual field 13 can be detected from the number and size of pulses in signal 10a. Signal processing portion 14 of light receiving mechanism 8 as shown in FIG. 8 is adapted to process signal 10a as described above.
In the process of detecting the particles as shown and described above, if the quantity of light received by light receiving mechanism 8 is insufficient, it is impossible to accurately detect the particles. Therefore, in general, the luminous intensity of the scattered light can be increased by narrowing the width of light beam 6 to increase the quantity of scattered light. In general, the diameter of light beam 6 in the vicinity of origin O is designed to be several tens of micrometers to several hundreds of micrometers. Focusing lens 7 shown in FIG. 8 is provided for this reason.
In the process of detecting the number and size of particles which pass visual field 13 as shown in FIGS. 8 and 9, if the distribution of the luminous intensity of light beam 6 is nonuniform in visual field 13, errors in detection will occur as will now be described. FIG. 10 is a diagram illustrating why errors in detection occur. FIG. 10A is a cross-sectional view of visual field 13 obtained by cutting through the portion of flow-cell 1 in the vicinity of visual field 13 as shown in FIG. 9 by the plane defined by axes A.sub.1 -A.sub.2 and C.sub.1 -C.sub.2. Reference numerals 15 and 16 respectively represent the visual field and light beam which correspond to visual field 13 and light beam 6 shown in FIG. 9. In this state, light beam 16 penetrates visual field 15. Reference numerals 17a and 17b represent two spherical particles having the same diameter which cross visual field 15 in the direction P as they are conveyed by sample fluid 2. With reference to FIG. 10A, when particles 17a and 17b move as described above, if the luminous intensity of light beam 16 has a spindle-shaped distribution with the maximum value of the luminous intensity at origin O as shown in FIG. 10B, the scattering of light from light beam 16 may not always occur in correspondence with the route of particles 17a and 17b passing through visual field 15. Furthermore, even if particles 17a and 17b are the same size, scattered light of differing luminous intensities may be produced. In the former case, an error in detecting the number of particles will occur, while in the latter case an error in detecting the size of the particles will occur. Since particle detecting devices as described above typically employ a laser beam as the light beam so that the size of even the smallest particles can be detected by increasing the luminous intensity of this light beam as it irradiates the sample fluid, such a laser beam usually has a bell-shaped luminous intensity distribution as shown in FIG. 10B and, consequently, errors in detection such as those described above may occur.
Therefore, a method is typically employed in which the flow channel of the sample fluid is narrowed to restrict fluid flow to the vicinity of axis A.sub.1 -A.sub.2 (FIG. 10A) wherein the luminous intensity is distributed uniformly, as shown by the luminous intensity curve of FIG. 10B. Alternatively and with reference to FIG. 9, a method is employed in which visual field 13 is disposed in the portion of flow-cell 1 in which the luminous intensity of light beam 6 is distributed substantially uniformly. Particle detecting devices employing the former method are unsatisfactory in that, if the luminous intensity of the light beam is increased by narrowing the beam width to enable the device to detect relatively smaller particles, the nonuniformity of the luminous intensity distribution in the sample fluid increases. As a result, the degree of error in detecting particle sizes increases or, in other words, the particle size resolving power of the detecting device deteriorates. When the flow field of the sample fluid is narrowed to restrict the sample fluid to the portion of flow-cell 1 in which the luminous intensity of the light beam has a uniform distribution, deterioration of resolving power can be avoided by reducing the quantity of fluid that can be measured in a predetermined time, e.g., by reducing the flow rate.
In detecting devices employing the latter method, when light beam 6 is narrowed to enable the device to detect smaller particles, the particle size resolving power of the device deterioriates for reasons similar to those described above. The luminous intensity is not distributed uniformly where visual field 13 of light receiving mechanism 8 is disposed in substantially the entire region of light beam 6. To prevent this deterioriation of resolving power, aperture 11a of aperture member 11 can be reduced in size to narrow visual field 13, thus reducing the quantity of light capable of passing through aperture 11a. As a result, the quantity of scattered light impinging upon photoelectric converter 10 may not increase even though the luminous intensity of light beam 6 is increased. Thus, the device cannot be adapted in this fashion to detect relatively smaller particles.